Employers often view labor from a short-run perspective. When more employees are needed, they can be hired; when fewer are needed, they can be laid off. The amount of labor hired would be determined by the price of output, the elasticity of demand, and the firm's productivity given its capital equipment. Economic theory suggests workers will be added until the added value of the additional output no longer exceeds the wage. The value of the output produced by hiring an additional worker (the amount of the product times the price) is called the marginal revenue product. If the demand curve shifts, its elasticity changes and employers would need more or fewer workers. Unions are interested in reducing employers' abil ity to lay off employees in response to demand reductions and to create rules for how layoffs would be implemented if they were necessary